When the biochemical analyzer analyzes body fluid, it first takes liquid samples. Driven by a motor, a pipetting probe is moved downward in a tube, and when the probe is in contact with the liquid surface, the motor must be stopped to prevent that the probe encounters the bottom of the tube and non-necessary body fluid is sucked. After sucked body fluid, driven by the motor, the probe is moved upward to return. Therefore, liquid level detecting must be carried out for biochemical analysis. When the probe is in contact with the liquid surface, a liquid surface contacting indication signal needs to be output to a control unit for controlling the motor, so as to control the motor to stop. The prior liquid level detecting technologies for the automatic biochemical analyzer mainly include a capacitance change type liquid level detecting, a pressure sensing type liquid level detecting, a wireless transmission and receiving type liquid level detecting and so on.
The capacitance change type liquid level detecting technology detects liquid level based on a change in capacitance of the pipetting probe, and is widely applied because of its characteristics such as high sensitivity, not affecting the accuracy of the sucked liquid samples and so on. In U.S. Pat. No. 6,107,810, a method for capacitance change type liquid level detecting is described, wherein, the pipetting probe is a single tube probe and is connected to a detecting circuit, the sample disk or reagent disk is grounded, and liquid level is detected by detecting a change in capacitance between the probe and the ground. The single tube probe for the method is susceptible to outside interference, and thus there is high requirement for the grounding of the sample disk.
A China patent, named “liquid level detector for biochemical analysis” (Patent No. CN02250812.0, publication date Nov. 26, 2003), disclosed an apparatus for detecting liquid level with capacitance change, wherein, the pipetting probe adopts a double-layer coaxial probe structure, the inner and outer probe tubes are made by a medically used stainless steel and equivalent to two electrodes of a capacitance, and an insulated casing is inserted between them. When the tip of the probe is in contact with the liquid level, the dielectric constant between the two electrodes varies and thus the capacitance of the probe is changed, consequently, the liquid level is detected by sensing the change in capacitance.
This apparatus generally comprises: a probe, an oscillation and frequency division circuit, a phase locked loop (PLL), an amplifier circuit, a comparison circuit and a control unit. The change in capacitance of the probe results in the change of oscillation frequency of the PLL and the change of oscillation frequency is converted into an electrical level signal. The electrical level signal is amplified by the amplifier circuit and the amplified signal is compared by the comparison circuit. Then, the comparison circuit outputs a signal to the control unit to control the motor to stop rotation. The pipetting probe then stops moving and goes back after sucking samples, and thus the object of the liquid level detecting is achieved.
However, this scheme for detecting liquid level is implemented by purely analog circuits, and a liquid level contacting indication signal will be output once the voltage value indicated by a current voltage signal is greater than a predetermined threshold. Therefore, this scheme has poor anti-interference performance, is susceptible to interference and causes false detection, and thus resulting in the control error for the motor. In order to improve the reliability of detection, namely the anti-interference performance, the only way to implement is by increasing the comparison threshold of the comparison circuit. However, simply increasing the comparison threshold results in the dropping of the sensitivity for one aspect, and needs a larger amount of reagents and samples for another aspect. With the development of the automatic biochemical analyzer technology and the environment protection requirements, laboratories are using less and less amount of reagents and samples, which set higher demands on detection sensitivity. However, the prior scheme for detecting liquid level is especially not suitable for detecting liquid level of a small amount of liquid.